Known brush assemblies are generally of two main types, either brushes mounted on leaf arms or guided brushes such as caged brushes. Leaf brushes are simple in construction but guided brushes are preferred when electrical noise is of concern. One source of electrical noise is the sideways floating and bouncing of the brush as it encounters irregularities in the brush path of a commutator or slipring. Leaf brushes continually change their position on the commutator surface. This is minimized by using a guided brush assembly.
Known guided brush assemblies for an electric motor having a commutator comprise a brush for sliding contact with the commutator, a brush terminal, a brush spring for urging the brush into contact with the commutator, and brush guiding means for guiding the brush towards the commutator.
Generally, the brush guiding means is a cage surrounding the brush on four sides and in which the brush is received to form a loose sliding fit. The cage has an open end adjacent the commutator and the brush spring pushes the brush away from the opposite end. A brush tail or shunt is required to electrically connect the brush to the brush terminal as neither the spring nor the cage makes satisfactory electrical contact with the brush, especially in a low voltage application.
Brushes for caged brush assemblies are usually long brushes. They are used in situations where substantial wear on the brush is expected over the life of the motor and the spring is long to provide a constant spring tension over the range of expected wear. The brush shunt is connected to the brush by being embedded into the end of the brush remote from the commutator (the non-working end). At the end of the brush life the shunt should not be exposed as contact with the commutator by the shunt may lead to damage of the commutator surface. This connection requires a significant portion of the brush. This is generally tolerated as the cage itself requires a minimum length of brush (dead length) to ensure avoidance of misalignment of the brush in the cage which could result in the brush becoming jammed and thus, not providing satisfactory contact with the commutator. As will be realized, such an arrangement requires an assembly of considerable size.
Therefore, in low noise, low wear situations, a caged brush assembly requires a brush length of excessive size compared with the expected wear. The normal leaf brush arrangement is not suitable for low noise situations because of the requirement to make the brush arm as long as possible to provide a constant spring force. Although this arrangement can satisfactorily handle short brushes, it is inherently unstable resulting in a significant amount of noise.
GB0345523A describes a guide brush assembly in which the brush is urged towards the commutator by a a coil spring located around a pin connecting the brush to the frame of the brush assembly. A bent plate spring is provided on the sides of the brush to prevent movement of the brushes circumferentially around the surface of the commutator. The construction is complicated requiring a coil spring, a brush with an embedded shunt, a pin fixed to the end of the brush, a brush assembly frame and a separate brush terminal.
Thus, there is a need for a brush assembly which can handle short brushes for use in low wear situations and provide a stable or moderately stable brush position.